Inhibition of Jak1 and Jak3 tyrosine phosphorylation and kinase activity and Stat5 phosphorylation by JAB/SOCS1/SSI-1.

293T⁺ cells were transiently transfected with the indicated amounts of Myc-tagged JAB/SOCS1/SSI-1 in the presence (+) or absence (−) of Jak1 or Jak3 cDNAs. Cells were lysed, and total cellular extracts (TCLs) were either directly analyzed on Western blots or were first immunoprecipitated with either (A) anti-Jak1 or (B) anti-Jak3. One-half of each immunoprecipitate was blotted with antiphosphotyrosine (αPY), anti-Jak1 or Jak3, and anti-Myc mAb 9E10 to detect JAB/SOCS1/SSI-1. The other half of each immunoprecipitate was subjected to in vitro kinase assay and then analyzed by SDS-PAGE and autoradiography.